Iris comes back with American star (6)
I believe the answer is:
sirius
'star' is the definition.
(Sirius is the dog star)
'iris comes back with american' is the wordplay.
'comes back' says the letters should be written backwards.
'with' is a charade indicator (letters next to each other).
'american' becomes 'US' (abbreviation for United States).
'iris' reversed gives 'siri'.
'siri'+'us'='SIRIUS'
(Other definitions for sirius that I've seen before include "The Dog Star, brightest of the fixed stars" , "The Dogstar" , "Prominent star" , "The Dog Star that follows the hunter Orion" , "Brightest star in the sky" .)